1. Field of the Invention
The present invention relates to an information storage system for recording and reproducing information by use of a light (or laser beam) and more specifically to an information storage system capable of recording new information without erasing previously recorded information (or direct overwrite) at the time of recording.
2. Description of Prior Art
An overwrite operation may be executed by modulating the light intensity at the time of recording into binary values of P.sub.H and P.sub.L for a recording medium capable of overwriting, as disclosed in the Journal of Magnetics Society of Japan, Vol. 15 Supplement No. S1, 1991, pp 293-298.
Normally, the value of P.sub.L is set at a value larger than the light intensity P.sub.R at the time of reproduction, such that at the time of designing a real information storage system the laser drive circuit must be so designed as to be capable of modulating the light intensity independently into three values, i.e., P.sub.H, P.sub.L and P.sub.R. In addition, in order to ensure a reliable recording operation, P.sub.H and P.sub.L values have to be established to attain an optimal condition.
A constitution of such a recording system according to a prior art is shown in FIG. 1. The information storage system shown in FIG. 1 is designed to detect the values of P.sub.H and P.sub.L and restore them to the original set values when the values of P.sub.H and P.sub.L which were set to an optimal condition are varied due to certain conditions, as explained below. In FIG. 1, the values of P.sub.H and P.sub.L (i.e., the values for the optimal conditions) set by a reference value setting circuit 5 are respectively transmitted to a laser drive circuit 3 through a P.sub.H value setting circuit 6 and a P.sub.L value setting circuit 7, respectively. The P.sub.H and P.sub.L values thus transmitted drive a laser diode 1 so as to modulate the laser light of the laser diode 1 into the light intensity of two levels corresponding to P.sub.H and P.sub.L in accordance with the binary information "a" from a data modulating circuit 4. The intensity of the light irradiated from the laser diode 1 is converted into electrical signals by a photodiode 2 for monitoring by a laser beam intensity monitoring circuit 8, and the P.sub.H and P.sub.L values are respectively monitored at a peak value detection circuit 9 and a bottom value detection circuit 12. The P.sub.H and P.sub.L values thus detected are compared by a comparison circuit 10 with the P.sub.H and P.sub.L values which have been set by the reference value setting circuit 5 and serve as the reference values. If there is any deviation from the reference values, such deviation is respectively compensated by a P.sub.H compensation circuit 11 and a P.sub.L compensation circuit 13, and the relative control signals are sent to the P.sub.H value setting circuit 6 and the P.sub.L value setting circuit 7 respectively, such that the compensated P.sub.H and P.sub.L values cause the laser diode 1 to be driven. Thus, the laser diode 1 is modulated by the desired light intensities of P.sub.H and P.sub.L values corresponding to the initial set values of P.sub.H and P.sub.L.
Since the storage system having a laser drive circuit as explained above according to a prior art controls P.sub.H and P.sub.L values independently, it is necessary to produce a recording medium having a wide range of laser beam intensity (or so-called power margin) so that the signal characteristics at the time of reproduction are not varied in relation to the optimal value at the time of recording even if the laser beam intensity may deviate from the optimal value set in advance for P.sub.H and P.sub.L respectively.
Namely, even if such a circuit as shown in FIG. 1 is used, which is adapted to compensate for the variation of the light intensities of P.sub.H and P.sub.L, the ultimate values of P.sub.H and P.sub.L are likely to deviate from the desired optimal values by .+-.10%, for example, due to variation of the application environment, such as variation of the application temperature between 0.degree. and 50.degree. C., variation of the irradiation characteristic of the laser diode 1 with age, variation of the light reception sensitivity of the photodiode 2 for monitoring and so forth. Furthermore, since the values of P.sub.H and P.sub.L are controlled independently, deviation of the P.sub.H and P.sub.L values from the optimal values occurs independently of each other. Suppose that, if the intensity of the laser diode 1 is varied to the optimal light intensity values of P.sub.H and P.sub.L by using signals with 50% duty, and the information which has been recorded on a recording medium by the intensity of the laser diode 1 varied as mentioned above is reproduced, the duty of the reproduced signal becomes 50%. In the above-mentioned case, the allowable deviation of the duty of the reproduced signal is in the order of .+-.4%, for example, in such an information storage system designed to attain a reproduced signal with 50% duty. As explained above, if the light intensity values of P.sub.H and P.sub.L deviate by .+-.10% independently from the respective desired optimum values, the deviation of duty of the reproduced signal exceeds the allowable range of .+-.4%. In view of this problem, there has been a need to produce a recording medium which attains in itself a wide power margin.
Furthermore, since the P.sub.L value is smaller than the P.sub.H value, it is difficult to control P.sub.L and P.sub.H with the same accuracy.